Mobile communication networks generally provide service in a very wide area, which is based on radio coverage offered by a terrestrial base station network or satellite repeaters. Base station systems or satellite terrestrial stations are connected to mobile services switching centres. A network operator may employ several interconnected mobile service switching centres. Furthermore, the mobile services switching centres must have connections to the public switched telephone network PSTN and to other mobile communication networks in various countries. Normally, interexchange digital connections are provided through which speech and data are transferred in 64 kbit/s PCM (Pulse Code Modulation) channels. These connections may be fixed or semi-fixed, or they may be established for each call individually. The fixed or semi-fixed connections are purchased and leased connections. In each case, it is important to the operators to optimize the utilization of the connection capacity and to minimize the costs arising therefrom.
One way of realizing this is to multiplex several calls into a PCM channel. Since there is a standard PCM interface between the exchanges, through which e.g. speech is transferred in the form of 64 kbit/s PCM samples, this requires speech compression, i.e. speech coding to a lower rate, and decompression at the ends of the connection. Compression equipment is complex and causes compatibility and signalling problems at the interexchange interfaces. Furthermore, speech coding to a low rate will inevitably deteriorate speech quality in comparison with 64 kbit/s PCM coding and is therefore not always an acceptable solution.
In digital mobile communication systems, also the entire speech and data transmission is digital. As far as the mobile communication network is concerned, the most limited resource is the radio path between the mobile stations and the base stations. To reduce the bandwidth requirements of a radio connection in the radio path, speech transmission utilizes speech encoding affording a lower transmission rate, for example 16 or 18 kbit/s instead of the 64 kbit/s transmission rate typically used in telephone networks. Speech encoding, which is based on representing speech as parameters, will be generally termed vo-coding (speech encoding) herein in distinction to PCM coding. Both sides of the radio interface, i.e. both the mobile station and the fixed network end, have a speech encoder and a speech decoder. The speech coder on the network side is sometimes also termed a transcoder. The transcoder may be accommodated in several alternative locations, for example at the base station or in the mobile services switching centre. The transcoder is often located remote from the base station in a `remote transcoder unit`. In the latter case, speech encoding parameters are sent between the base station and the transcoder unit in specific frames.
In each mobile terminating or mobile originating speech call, a transcoder is connected to the speech connection on the network side. The interface of the transcoder towards the mobile services switching centre is 64 kbit/s. The transcoder decodes the mobile originating speech signal (uplink direction), vocoded into a 8/16 kbit/s transmission channel, to a rate of 64 kbit/s, and encodes the mobile terminating 64 kbit/s speech signal (downlink direction) to a rate of 8/16 kbit/s. Hence, also speech quality is lower than in a normal telephone network.
However, compression equipment at interexchange connections would further degrade speech quality. This is due to tandem speech coding: encoding at the mobile station, decoding in the transcoder, encoding in first compression equipment, and decoding in second compression equipment. In such a case, one party of the call is a mobile station and the other a subscriber of a public switched telephone network (PSTN), for instance.
If the call takes place between two mobile stations (Mobile to Mobile Call, MMC), even multiple speech coding may be performed. In such a case, there is a transcoder on the connection between the calling mobile station and the mobile services switching centre, and, correspondingly, a second transcoder between the called mobile subscriber and the (same or another) mobile services switching centre. These transcoders are then interconnected through the connection between the mobile services switching centres as a result of normal call switching. In other words, for each MMC call there are two transcoder units in series connection, and speech encoding and decoding are performed twice for the call. So far, tandem coding has not been a major problem because relatively few calls have been MMC calls. However, as the number of mobile stations increases, the number of MMC calls will also be higher and higher. One speech compression operation on the interexchange connection would worsen the situation by increasing the number of speech coding operations to three. If the call has to be routed via several mobile services switching centres and via compressed intermediate connections, the number of speech encoding operations may be multiplied.